Method of forming diamond-like carbon coating in vacuum

ABSTRACT

A method of forming a diamond-like carbon coating in vacuum, comprising the steps of: pretreatment of the surface of the pad; placing the part into a vacuum chamber; treating the surface of the part with accelerated ions; applying a sublayer of a material onto the treated surface of the part; electric are vacuum sputtering a graphite cathode from a cathode spot and producing a carbon plasma accelerating an ion component of the carbon plasma; depositing the produced carbon plasma on the surface of the part and producing the diamond-like carbon coating. A pulsed electric are discharge is used, by which a plurality of cathode spots are excited at the end surface of the graphite cathode, cathode spots moving along the end surface of the cathode at a speed of from 10 to 30 m/s and generating a carbon plasma having an ion energy of 40 to 100 eV and an ion concentration in the plasma of 10 12  to 10 14  cm −3 , with electrically insulating the part in the vacuum chamber. The temperature of the part in the range of 200 to 450 K by controlling the repetition frequency of discharge pulses basis.

CROSS REFERENCE TO RELATED APPLICATION

The present application is the national stage under 35 U.S.C. 371 ofPCT/NO98/00158, filed May 28, 1998.

The present invention relates to the field of producing superhardwear-resistant coatings in vacuum, and more specifically to a method offorming a diamond-like carbon coating in vacuum.

The invention may be used to increase the service life of cutting tools,measuring tools, friction assemblies and machine parts as well as in themedicine to improve the biological compatibility of implants, in theelectronic engineering to increase the service life of audio and videoheads, to improve characteristics of acoustic membranes, as coatings foroptical parts, and as decorative coatings.

Known in the art is a method of producing high-hardness diamond-likecarbon coatings on metal and dielectric substrates (cf., SU Inventor'sCertificate No. 411037, 1975) wherein graphite is cathode-sputtered in amagnetic field at a low pressure of a noble gas, krypton, of 10⁻⁵ to10⁻² Pa to a cooled substrate having a temperature less than 100 K.

Said process has low productivity due to the low pressure of theprocessing gas, krypton, and low energetic characteristics of the glowdischarge at this pressure. It is technologically difficult to maintainsuch low temperature of the parts to be treated. One has to complicatethe processing equipment significantly to achieve ultrahigh vacuum.

The closest technical solution is a method of forming a diamond-likecarbon coating in vacuum, comprising the steps of pretreatment of thesurface of a part, placing the part into a vacuum chamber, treating thesurface of the part with accelerated ions, applying a sublayer of amaterial onto the surface of the part, electric arc vacuum sputtering agraphite cathode from a cathode spot and producing a carbon plasma,accelerating an ion component of the carbon plasma, depositing theproduced carbon plasma on the surface of the part and producing thediamond-like carbon coating (cf., D. R. McKenzie et al. “Properties oftetrahedral amorphous carbon prepared by vacuum arc deposition,” Journal“Diamond and Related Materials,” 1, 1991, p. 51-59).

In said method the sputtering of the cathode is performed in astationary discharge from one cathode spot, the carbon plasma isproduced, the plasma is separated, that is, cleaned from microparticlesbeing formed in a stationary cathode spot. The ion component of theplasma is electrostatically accelerated by supplying a negative,high-frequency potential, and the diamond-like carbon coating isproduced.

In said method the use of the stationary discharge does not permit toobtain the plasma energy necessary to form the diamond like carboncoating, that implies the necessity of additional accelerating theplasma ions by supplying a potential to the part. It results in heatingthe coating and deterioration of its properties, that is, decreasedmicrohardness.

In addition, there arises a hazard of overheating small-size parts aswell as sharp edges, that leads to their softening. If the part is madeof a dielectric material the electrostatic acceleration has a smalleffect.

The stationary electric arc discharge is defined by presence of amovable cathode spot being a source of a low energy carbon plasma aswell as a great number of hard graphite fragments escaping from thecathode spot. The ion energy does not exceed 10 to 15 eV. When strikingthe surface of the part, the graphite fragments considerably deterioratethe quality of the produced coating.

To eliminate this disadvantage, said method uses a curvilinear magneticdeflection system that significantly complicates the method of producingthe coating. In addition, the stationary cathode spot being the sourceof the carbon plasma in said method, produces a narrow carbon plasmabeam that does not permit application of a uniform diamond-like carboncoating on elongated parts.

The comparatively small plasma density, that is, the ion concentrationimplies enhanced requirements to the vacuum level to avoid contaminationof the coating with residual gases and respective deterioration of thecoating quality. The stationary nature of the process complicates themethod of applying the coating because it becomes difficult to maintainthe necessary temperature mode. The application of the diamond-likecarbon coating onto small-size and film materials involves severedifficulties; in this case the coating properties are unstable.

A foundation of the present invention is the problem of creating amethod of forming a diamond-like carbon coating in vacuum, wherein theuse of a pulsed arc discharge for generating a plurality of cathodespots at the end surface of the cathode as well as maintaining thetemperature of the part by means of changing a pulse repetitionfrequency will allow to simplify the method of forming the coating, toimprove its stability and productivity, and to improve the quality ofthe formed coating, in particular, its uniformity and wear resistance.

The posed problem is solved by that a method of forming a diamond-likecarbon coating in vacuum, comprising the steps of pretreatment of thesurface of the part, placing the part into a vacuum chamber, treatingthe surface of the part with accelerated ions, applying a sublayer of amaterial onto the treated surface, electric arc vacuum sputtering agraphite cathode from a cathode spot and producing a carbon plasma,accelerating an ion component of the carbon plasma, depositing theproduced carbon plasma on the surface of the part and producing thediamond-like carbon coating. According to the invention and in order toproduce, accelerate and deposit the carbon plasma, said methodcomprising the steps of using a pulsed electric arc discharge by which aplurality of cathode spots are excited at the end surface of thegraphite cathode, said spots moving along the end surface of the cathodeat a velocity of 10 to 30 m/s and generating the carbon plasma having anion energy of 40 to 100 eV and a concentration of ions in the plasma of10¹² to 10¹⁴ cm⁻³, with the part being electrically insulated in thevacuum chamber, and maintaining the temperature of the part in the range200 to 450 K by controlling the discharge pulse repetition frequency.

It is helpful to use metal ions as the accelerated ions in treating ametal part.

It is expedient to use as the sublayer material a metal of thickness 100to 500 A; used for said purpose was a metal selected from the groupconsisting of titanium. chromium, molybdenum, zirconium, niobium,tungsten.

It is advantageous to increase the temperature of the part up to a valuein the range of 473 to 573 K in treating the surface of the part withthe accelerated ions of a metal and then to cool the part to atemperature of 293 to 300 K and to re-treat the surface of the part withaccelerated ions of a metal until the temperature reaches 323 K.

It is expedient to perform the method in an argon atmosphere at apressure of 10⁻² to 10⁻¹ Pa.

It is helpful to use ions of a gas as the accelerated ions in treating adielectric part, said gas being selected from the group consisting ofargon, nitrogen, oxygen or a mixture thereof.

It is advantageous to apply a sublayer of aluminum nitride of thickness50 to 200 A to a glass part in treating thereof.

It is expedient to use as the graphite cathode a highly purifiedgraphite wherein the pore amount is about 0.5%.

It is helpful to use as the graphite cathode a graphite with anadmixture of a doping element which is an element selected from thegroup consisting of silicon, germanium, osmium, bismuth, phosphorus, andantimony.

It is advantageous to sputter an additional cathode made of a metalselected from the group consisting of titanium, chromium, aluminiumn,zirconium, silicon, germanium.

It is also helpful to treat the diamond-like carbon coating formed onthe part with accelerated ions of a gas or a metal.

The invention will now be described in detail with reference to variousspecific embodiments thereof.

The method of forming a diamond-like carbon coating in vacuum isperformed as follows.

The surface of a part is mechanically prepared and then degreased. Afterthat the part is placed into a special fixture in a vacuum chamber andfastened. The arc current is set to be 60 to 80 A, a negative potentialof 1000 to 1500 V is being supplied to the part. In this manner thetreating with accelerated ions is performed.

Then the potential to be supplied to the part is lowered to 100 V and asublayer of a metal of thickness 100 to 500 A is applied to the treatedsurface. It is possible to use a metal selected from the groupconsisting of titanium, chromium, molybdenum, zirconium, niobium,tungsten.

Then the electric arc vacuum sputtering of the graphite cathode isperformed and the carbon plasma is produced. Used for this purpose isthe pulsed electric arc discharge that has the following parameters: thevoltage in a capacitor battery of a capacitance of 2000 μF is 300 V; thedischarge time is 0,5 ms; the repetition frequency of pulses is from 1to 20 Hz. Under such conditions a plurality of cathode spots is excitedat the end surface of the graphite cathode. Said cathode spots movealong the end surface of the cathode at a speed of 10 to 30 m/s andgenerate a carbon plasma having an ion energy of 40 to 100 eV and an ionconcentration in the plasma of 10¹² to 10¹⁴ cm ⁻³. As this takes place,a potential is not supplied to the part and the part itself is insulatedfrom all electrodes and the housing of the vacuum chamber.

The temperature of the part is maintained in the range of 200 to 450 Kby controlling the repetition frequency of discharge pulses.

The produced carbon plasma is deposited on the surface of the part andthe diamond-like carbon coating is produced.

If it is found, visually or under a microscope, after the preliminarytreating, that the treating was ineffective and oxide films remain onthe surface of the part, then, the duration of ion treating of thesurface with accelerated ions of a metal is increased, the temperatureof the part being increased to the value of 473 to 573 K. Then the partis cooled to a temperature in the range of 293 to 300 K. The surface ofthe part is re-treated with accelerated ions of a metal until thetemperature reaches 323 K.

To improve the intensity of purification, the ion treating is performedin an argon atmosphere at a pressure of 10⁻² to 10⁻¹ Pa.

In treating a dielectric part, ions of a gas are used as the acceleratedions, said gas being selected from the group consisting of argon,nitrogen, oxygen or a mixture thereof.

In treating a glass part, after gas ion treating, applied to the glassis an aluminum nitride layer of thickness 50 to 200 A to enhanceadhesion between the diamond-like carbon coating and the surface of theglass part.

Used as the graphite cathode in said method is a highly purifiedgraphite wherein the pore amount is about 0.5%. To improve the qualityof the diamond-like carbon coating, used is a highly purified graphitehaving a minimum pore amount because the pores enclose such impuritiesas gaseous nitrogen, oxygen, water vapour. When penetrating into thecoating to be formed the impurities decrease its quality.

To obtain semiconducting properties of the diamond-like carbon coating,used as the graphite cathode is a graphite having an admixture of adoping element which is an element selected from the group consisting ofsilicon, germanium, osmium, bismuth, phosphorus, antimony.

If a diamond-like carbon coating with a different value of electricalresistance is needed, an additional cathode is sputtered, said cathodebeing made of a metal selected from the group consisting of titanium,chromium, aluminum, zirconium, silicon and germanium.

To change optical and electrical characteristics as well as to obtain apattern on the coating, the diamond-like carbon coating formed on thepart is treated with accelerated ions of a gas or a metal.

EXAMPLE 1

Used was a polished sample of a hardened carbon steel having dimensionsof 20×20×20 mm, said sample was fastened in a special fixture, placedinto the vacuum chamber, and the chamber was evacuated to a pressure of5×10⁻³ Pa. The ion treating was performed with titanium ions generatedby the electric arc plasma source having a titanium cathode. A negativepotential of 1000 V was supplied to the sample. An arc current of 80 Awas set. The treating time was 5 minutes. Then the potential was loweredto 100 V and a titanium sublayer of 200 A was applied. Then a diamondlike carbon coating of 10 micrometers was applied by electrical arcsputtering the graphite cathode in the pulsed discharge withoutsupplying a potential to the sample and with the following parameters:the voltage in the capacitor battery of a capacitance of 2000μF was 300V; the discharge time was 0,5 ms; the repetition frequency was 10 Hz. Asthis took place, the ion energy was 70 V, the plasma density was 1×10¹³cm⁻³. The temperature of the sample was increased to 423 K.

Graphite impurities in the coating were not found by the ESCA (electronspectral chemical analysis) method.

The microhardness of the diamond-like carbon coating was 8000 HV at aload of 100 g. The coefficient of friction on titanium nitride was 0.04,the coefficient of friction on a hardened steel was 0.08, thecoefficient of friction on copper was 0.1.

It was established by X-ray analysis that the coating was amorphous.

EXAMPLE 2

Used was an artificial heart valve made of titanium, said valve wasfastened in a special fixture, placed into the vacuum chamber, and thechamber was evacuated to a pressure of 5×10⁻³ Pa. The ion treating wasperformed with titanium ions generated by an electric arc plasma sourcehaving a titanium cathode. A negative potential of 1000 V was suppliedto the valve. An arc current of 80 A was set. The treating time was 5minutes. After that the potential of the valve was lowered to 100 V anda titanium sublayer of thickness 500 A was applied. Then a diamond-likecarbon coating of 2 micrometers was applied by electrical arc sputteringthe graphite cathode in the pulsed discharge without supplying apotential to the valve and with the following parameters: the voltage inthe capacitor battery of a capacitance of 2000 μF was 300 V; thedischarge time was 0,5 ms; the repetition frequency was 3 Hz. As thistook place, the ion energy was 70 V, the plasma density was 1×10¹³ cm⁻³.The temperature of the valve was increased up to 423 K.

It was established by X-ray analysis that the coating was amorphous.

It was established by medical and biological studies that the coatinghad satisfactory biological compatibility properties.

EXAMPLE 3

Used was a cutting plate made of a hard alloy for treating light-weightaluminum-based alloys. Said plate was fastened in a special fixture,placed into the vacuum chamber, and the chamber was evacuated to apressure of 5×10⁻³ Pa. The ion treating was performed with titanium ionsgenerated by an electric arc plasma source having a titanium cathode. Anegative potential of 1500 V was supplied to the plate. An arc currentof 80 A was set. The treating time was 5 minutes. Then the plate wascooled to a temperature of 300 K. The ion treatment was repeated for 1minute. After that the potential of the plate was lowered to 100 V and atitanium sublayer of 200 A was applied. Then a diamond-like carboncoating of 2 micrometers was applied by electrical arc sputtering thegraphite cathode in the pulsed discharge without supplying a potentialto the plate and with the following parameters: the voltage in thecapacitor battery of a capacitance of 2000 μF was 300 V; the dischargetime was 0,5 ms; the repetition frequency was 10 Hz. As this took place,the ion energy was 70 V, the plasma density was 1×10¹³ cm⁻³. Thetemperature of the plate was increased up to 423 K.

The microhardness of the diamond-like carbon coating was 8000 HV at aload of 100 g. The coefficient of friction on aluminum was 0.12.

The production test of the hard alloy plate, performed under conditionsof full-scale production of an automobile plate, has shown an increasein its service life and improved surface quality.

It was established by X-ray analysis that the coating was amorphous.

What is claimed is:
 1. In a method of forming a diamond-like carboncoating in a vacuum, comprising: pretreatment of a surface of a part tobe coated; placing the part into a vacuum chamber; treating the surfaceof the part with accelerated ions; applying a sublayer of a materialonto the treated surface of the part; electric arc vacuum sputtering agraphite cathode from a cathode spot and producing a carbon plasma;accelerating an ion component of the carbon plasma; depositing theproduced carbon plasma onto the surface of the part and thus producingthe diamond-like carbon coating; the improvement wherein in order toproduce, accelerate and deposit the carbon plasma, said methodcomprising using a pulsed electric arc discharge at a repititionfrequency by which a plurality of cathode spots are excited at an endsurface of the graphite cathode, said cathode spots moving in the endsurface of the graphite cathode at a speed of from 10 to 30 ms andgenerating a carbon plasma having an ion energy of 40 to 100 eV and anion concentration in the plasma of 10¹² to 10¹⁴ cm⁻³, with the partbeing electrically insulated in the vacuum chamber, and maintaining atemperature of the part in the range of 200 to 450 K by controlling therepetition frequency of discharge pulses.
 2. A method according to claim1, characterized in using metal ions as the accelerating ions intreating a metal part.
 3. A method, according to claim 2, characterizedin using as the sublayer material a metal selected from the groupconsisting of titanium, chromium, molybdenum, zirconium, niobium, andtungsten, said metal sublayer having a thickness of 100 to 500 A.
 4. Amethod according to claim 3, characterized in increasing the temperatureof the part from 473 to 573 K in treating the surface of the part;subsequent cooling the temperature of the part from 293 to 300 K;re-treating the surface of the part with accelerated ions of a metaluntil the temperature reaches 323 K.
 5. A method according to claim 4,characterized in that the method is performed in an argon atmosphere ata pressure of 10⁻² to 10⁻¹ Pa.
 6. A method according to claim 3,characterized in that the method is performed in an argon atmosphere ata pressure of 10⁻² to 10⁻¹ Pa.
 7. A method according to claim 2,characterized in increasing the temperature of the part from 473 to 573K in treating the surface of the part; subsequent cooling thetemperature of the part from 293 to 300 K; re-treating the surface ofthe part with accelerated ions of a metal until the temperature reaches323 K.
 8. A method according to claim 7, characterized in that themethod is performed in an argon atmosphere at a pressure of 10⁻² to 10⁻¹Pa.
 9. A method according to claim 2, characterized in that the methodis performed in an argon atmosphere at a pressure of 10⁻² to 10⁻¹ Pa.10. A method according to claim 1, characterized in using gas ionsselected from the group consisting of argon ions, nitrogen ions, oxygenions and mixture thereof, said gas ions comprising the accelerating ionsin treating a dielectric part.
 11. A method according to claim 1,characterized in preliminary applying an aluminum nitride layer ofthickness 50 to 200 A to a glass part in treating thereof.
 12. A methodaccording to claim 1, characterized in using as the graphite cathode apurified graphite having a pore amount of about 0.5%.
 13. A methodaccording to claim 1, characterized in using as the graphite cathode agraphite having an admixture of a doping element which is an elementselected from the group consisting of silicon, germanium, osmium,bismuth, phosphorus, antimony.
 14. A method according to claim 1,characterized in sputtering an additional cathode made of a materialselected from the group consisting of titanium, chromium, aluminum,zirconium, silicon and germanium.
 15. A method according to claim 1,characterized in treating the diamond-like carbon coating formed on thepart with accelerated ions of a gas or a metal.
 16. A method of forminga diamond-like carbon coating on a substrate, comprising: treating asurface of the substrate to be coated with accelerated metal ions in avacuum; applying a sublayer of a material onto the treated surface ofthe substrate in said vacuum; electric arc sputtering a graphite cathodein said vacuum from a cathode spot and producing a carbon plasma using apulsed electric arc discharge by which a plurality of cathode spots areexcited at an end surface of the graphite cathode whereby said cathodespots move along the end surface of the graphite cathode at a speed offrom 10-30 m/s and said carbon plasma has an ion energy of 40-100 eV andan ion concentration in the plasma of 10¹² to 10¹⁴ cm⁻³; maintaining atemperature of said substrate of 200-400 K while maintaining saidsubstrate electrically insulated under said vacuum; and depositing theproduced carbon plasma in said vacuum onto said surface of saidsubstrate to produce said diamond-like carbon coating.